1. Field of the Invention
The present invention relates to an optical device and a lens unit, each containing an insulating liquid and an electroconductive liquid which are mutually immiscible and optically transparent and have different refractive indices. The present invention also relates to an imaging apparatus which takes pictures by capturing object light coming through an optical shooting system including the optical device.
2. Description of the Related Art
There are some types of cameras, such as electronic still cameras and film cameras. The electronic still camera forms an image of an object on a CCD (charge coupled device) solid imaging device and captures the signals of the image. The film camera takes pictures on a photographic film. Some of these cameras have a zooming mechanism for freely setting a shooting view angle. Such a camera includes a shooting lens whose focal length is varied according to the operation of a zoom switch. The shooting lens is generally composed of some lens elements, that is, it is a compound lens, and the relative positions of the lens elements are adjusted according to the focal length set by the zoom switch. This type of camera includes a cam mechanism. The cam mechanism transfers the rotation of a motor according to the operation of the zoom switch so that the lens elements are independently shifted along the optical axis direction to adjust their relative positions, thereby changing the focal length.
The lens elements include a focus lens for adjusting the focus. A lens driving mechanism for moving the focus lens may be provided separately from the cam mechanism.
As an alternative to the shooting lens having driving mechanisms, a liquid or fluid lens has recently been proposed which contains mutually immiscible two types of liquid having different refractive indices and whose focal length is variable, by, for example, “Philips' Fluid Lenses” of Royal Philips Electronics, dated Mar. 3, 2004 on a website.
This liquid lens contains mutually immiscible two types of liquid having different refractive indices. One of the liquids is electrically conductive aqueous solution, and the other is an electrically insulating oil. The two liquids are contained in a liquid container made of a short tube whose ends are closed with optically transparent end caps. The inner wall of the tube and the inner wall of one of the end caps are coated with a water-repellent film. In the liquid lens having such a structure, the inner walls coated with the water-repellent film of the tube and one end cap repel the electroconductive aqueous solution, so that the electroconductive aqueous solution comes into contact with the other end cap, forming a hemispherical shape. Consequently, the interface between the electroconductive aqueous solution and the insulating oil serves as a concave lens. The liquid lens also includes two electrodes for applying a voltage to the electroconductive aqueous solution. One of the two electrodes is in contact with the electroconductive aqueous solution and the other is provided at the rear of the water-repellent film. When a voltage is applied between the electrodes, electrical charges are released to the electroconductive aqueous solution from the electrode in contact with the electroconductive aqueous solution. The released charges stay at the interface between the electroconductive aqueous solution and the insulating oil. The charges at the interface and the opposite-polarity charges built up on the electrode at the rear of the water-repellent film are attracted to each other by Coulomb force, and the charges in the electroconductive aqueous solution are drawn to the vicinity of the water-repellent film. Consequently, the electroconductive aqueous solution wets the water-repellent film coating the inner wall of the tube to change the shape of the interface between the two liquids. Specifically, as the voltage applied to the electroconductive aqueous solution is increased, the curvature radius of the interface between the electroconductive aqueous solution and the insulating oil, which has served as a concave lens initially, is changed, so that, for example, the shape of the interface becomes completely flat or the electroconductive aqueous solution is allowed to serve as a convex lens. Thus, the focal length can be varied.
If a voltage is repeatedly applied between the two electrodes of the liquid lens so that electrical charges are repeatedly released to the electroconductive aqueous solution, the electroconductive aqueous solution may be electrolyzed to generate hydrogen and oxygen and, thus, allow bubbles to be trapped in the liquid container disadvantageously. The bubbles diffuse light coming into the liquid lens, thus degrading the performance of the lens. The bubbles may also damage the liquid container, depending on where the bubbles are generated.
These problems can occur not only in the liquid lens, but also in the cases where the principle of the liquid lens that the shape of the interface is changed is applied to other optical devices.